The invention pertains to a pressure filter for liquids.
In particular, the invention pertains to a pressure filter for liquids that among others is utilized for technical liquids to separate a solid phase from a liquid phase. This may involve the treatment of process water for the purpose of water purification. The required high throughput volume is, with the pressure filter according to, the invention essentially based on the positive pressure maintained in the pressure vessel, thereby raising the pressure differential in the filters. This causes rapid growth of the filter cake formed by the solid phase of the solids suspension in the filter medium, as soon as liquid flows through the affected filter from the outside in.
The comparatively high throughput of such liquid filters is based on continuous filtration. It follows from this that the filter elements, which are moved stepwise in the pressure vessel during the rotation along a circular path, are in the blow-off station individually freed of the filter cake adhering to its filter medium, while the other filter elements continue the filtration or are being prepared for filtration. Another advantage of such pressure filters for liquids is that the filter cake is obtained relatively dry, since the drying process starts after the emergence of the filter elements and continues until the throw-off of the filter cake. This simplifies the further treatment of the filter cake substantially. Therefore, such filter cakes are also suited for the severely contaminated liquids with difficult to dewater filter cakes.
These and other advantages of the pressure filter according to the invention can be achieved with a control that regulates the filtration process steps resulting from the functionality of the pressure filter. The stepwise controllable rotational motion of the filter elements lets these dip individually into the solids suspension and move through the liquid to be filtered, until they finally emerge again and reach the dry sphere above the surface of the liquid. The pressure differential in the filters is maintained here and pushes in this way further liquid through the filter cake as filtrate that is discharged from the filter. The removal of the finally dried filter cake from the surface of the filter medium of each filter cartridge (filter element) occurs by a blast of air that reverses the pressure differential in the filter as and such throws off the filter cake from the filter medium. The filter cake falls onto a filter cake transporting device which, e.g., consists of a chain conveyor and is discharged through a pressure lock, so that the working pressure in the pressure filter does not have to be lowered.
Pressure filters of this type are known in principle and have so far been applied for the filtration of coal slurries (ref. periodical "Aufbereitungstechnik" 27, 1986, pp 387/395). In these, the filters consist in each case of a multitude of disk-like filter elements arranged on a common axis around which the filter elements rotate in a pressure housing, together and counter to the rotational motion of the filter. Although the mechanical requirements for filters of such design are comparatively high, in spite of the disk form of the filter elements, the filter area in total cannot be increased at will. This is so, because on one hand, depending on the individual case, the thickness of the filter cake reaching the blow-off station predetermines the axial distance of the filter elements that in the blow-off stations must still be sufficiently large to assure a successful blow-off. On the other hand, however, the disk diameter cannot be increased at will, since it determines the number of the filters to be housed in the pressure housing and therewith the number of the filtration processes during one revolution of the filter. Generally speaking, the disk form of the filter creates the difficulty in pushing the filter cake off without leaving a residue, since the gaps between the filter elements must naturally be chosen as small as possible.